This proposal is designed to test the hypothesis that binding of surfactant-associated glycoprotein, SP-A, to pulmonary type II epithelial cells and its subsequent receptor mediated endocytosis, mediates the effects of SP-A on surfactant phospholipid homeostasis in adult and developing Type II cells. We hypothesize that SP-A binds to a specific surface receptor on Type II cells and that this receptor appears during perinatal lung maturation. Binding of SP- A to Type II cells will be characterized morphologically using native SP-A in conjunction with an immunogold bridging technique. Internalization and intracellular trafficking of SP-A will be visually monitored using a bioactive biotinyl probe in association with streptavidin-gold as the electron dense marker. Characteristics of SP-A binding to the Type II cell surface will be correlated with its inhibitory effects on phospholipid secretion and its stimulatory effects on phospholipid reuptake. The biochemical determinants of the SP-A interaction with the binding site will be determined after modification of cell surface protein and carbohydrate. The ontogenic expression of SP-A and endocytosis will be determined in fetal rat Type II epithelial cells and the effects of hormones which induce Type II cell differentiation on the appearance and function of the SP-A binding sites will be clarified. These studies are intended to define mechanisms of action of SP-A which may mediate surfactant homeostasis and surfactant function during perinatal development. It is hoped that these studies will provide important information related to therapy of hyaline membrane disease in premature infants and in other surfactant deficient states.